As far as I know, you have to use the “Undo” button to restore the original file. It will undo all changes made after the fade out, so apply the fade (or whatever you want) as the last step.
But if you close and re-open the project, you can’t undo the changes. You have to reload the original audio file.
So, I suggest you to edit the audio event in the sequencer as much as possible and not in the audio editor. Moreover, this allows you to choose among different fade curves (right click on fade grip to display the menu).